Early organic light emitting devices were fabricated by deposition of metallic electrodes on either side of a thin organic crystal, for example single crystal anthracene as described in U.S. Pat. No. 3,530,325. Because of the relatively large thickness of the crystal, and the requirement for application of high electric fields across the organic material, this resulted in the need for high voltage operation of the devices and therefore a reduction of the power efficiency of the devices. Also this approach is expensive due to the difficulties in fabricating thin crystal layers. As a result of these problems, successful attempts were made to deposit thin film organic molecular and oligomer materials by vacuum sublimation onto a substrate which already has an electrode coating. Reference is made to C. W. Tang, S. A. Van Slyke and C. H. Chen, J. Appl. Phys. 65, 3610 (1989) which describes the deposition of charge transport and emitting layers to make a light emitting device. Subsequent to deposition of the charge transport and emitting layers, a top metal electrode can be deposited by a vacuum coating technique (e.g. evaporation). The layers have a strict uniformity requirement that may be difficult to satisfy over large coating areas. However, the main disadvantage of this technique is the need to deposit the metal electrode by vacuum metallisation.
EP-A-331997 describes the manufacturing technique for an electroluminescent element in which the luminescent layer contains fluorescent material such as for example zinc sulphide. In that manufacturing technique, a back electrode is formed of an aluminium foil and carries an insulating layer and a luminescent layer. This composite is laminated to a transparent conductive film in the form of a continuous web.
The discovery that conjugated polymers could be made to exhibit electroluminescence, as described in our earlier U.S. Pat. No. 5,247,190 (herein incorporated by reference), allowed for different deposition techniques, namely the use of wet-film coating techniques such as spin-coating. These films can have better thermal and mechanical stability than sublimed molecular films. The deposition of a plurality of layers, however, requires careful selection of solvents and polymer systems to avoid redissolution of the previously deposited layers. Alternatively, soluble precursor polymers are deposited as wet films and subsequently converted (e.g. by heat treatment) to the final insoluble polymer form. The heat treatment has to avoid damage to any previous layers or substrates--in particular plastic substrates that are sued for flexibility and/or transparency can be prone to damage. One way around this is to deposit the precursor material on a flexible, high temperature resistance material such as a metal foil and then to deposit a transparent electrode after conversion. In this case the deposition conditions have to be carefully considered to allow a high quality transparent and conductive film to be produced.
One aim of the invention is to develop a technique for fabricating a multilayer organic light emitting device which avoids these problems.